Document processing devices include printers, copiers, scanners and e-mail gateways. More recently, devices employing two or more of these functions are found in office environments. These devices are referred to as multifunction peripherals (MFPs) or multifunction devices (MFDs). As used herein, MFP means any of the forgoing.
Toner-based print engines of MFPs utilize a transfer belt unit (TBU) as part of the printing function. Toner is selectively attracted onto one or more photoconductive drums of an electrostatic process unit (EPU) in accordance with an image to be printed. The transfer belt transfers the toner from the photoconductive drums onto the paper, after which the transferred toner is then fused by heat onto the paper and delivered to a tray for retrieval by a user.
In black and white printers, a single photoconductive drum is used, while in color printers four or more photoconductive drums are used. Each photoconductive drum successively places toner of a particular color, such as yellow, magenta, cyan, or black, onto the transfer belt in accordance with the image to be printed. After the transfer belt has passed each photoconductive drum, the transfer belt has the entire image to be printer which is then transferred to the paper and fused to the paper by heat.
In order to transfer the toner between a photoconductive drum and the transfer belt, the transfer belt is electrically charged and brought within close proximity to the photoconductive drum so that toner from the photoconductive drum is attracted to the transfer belt. To maintain positional accuracy of the transfer belt relative to the photoconductive drum, a transfer roller is placed against the transfer belt opposite to the photoconductive drum. Each transfer roller ensures that the transfer belt is close enough to the photoconductive drum to allow most, if not all, of the toner from the photoconductive drum to move onto the transfer belt.
However, transfer rollers require periodic maintenance as bearings can wear or become clogged with stray toner. Also, because each transfer roller has a fixed diameter, the placement of multiple transfer rollers in a color printer places design constraints on the printer which must accommodate all of the transfer rollers. Smaller transfer rollers can reduce design constraints. However, smaller transfer rollers have surfaces with greater arcs than larger transfer rollers. Smaller transfer rollers can reduce the area of the transfer belt that is immediately proximate to each photoconductive drum, which also has an arced surface, which can affect the effectiveness of the transfer of toner from the photoconductive drum to the transfer belt.